1. Field of the Invention
The invention relates generally to the non-invasive diagnosis of conditions within a human or animal body and, more particularly, the invention relates to a diagnostic system and techniques that use the acoustic characteristics within a body to detect the location of an endotracheal tube therein.
2. Description of Related Technology
One particularly problematic respiratory condition is pneumothorax. Generally speaking, pneumothorax refers to the formation of a gas cavity between one or both lungs and the chest wall. As is well known, pneumothorax has many potential causes, including, for example, spontaneous rupture of small alveoli or blebs, progression of inflammatory diseases, complications of diagnostic or therapeutic procedures, penetrating wounds caused by a knife, bullet, etc. and blunt chest trauma, which may be, for example, caused by motor vehicle accidents. Although trauma is a significant cause of pneumothorax, severe chest wall injury is often difficult to detect based on the outward appearance of a patient's body and, as a result, the diagnosis of pneumothorax is often missed in these cases.
Pneumothorax also occurs in 5-15% of mechanically ventilated patients, and other iatrogenic pneumothoraces are becoming more common with the increasing use of chest invasive procedures such as central venous line insertions, which are often used for monitoring and fluid replacement in emergency trauma cases, and percutaneous transthoracic lung biopsies. For these invasive procedures, the pneumothorax rates are about 5% and 20%, respectively. It is estimated that over 50,000 cases of pneumothorax occur each year in the United States and, thus, more effective diagnosis of pneumothorax could significantly reduce morbidity and mortality.
Conventional pneumothorax diagnostic techniques are typically based on patient history, physical examination of the patient, chest x-rays (CXRs), computerized tomogram (CT) and ultrasound. Patient history, physical examination and CXRs are the techniques most commonly employed to diagnose pneumothorax. Unfortunately, patient history and physical examination are typically unreliable techniques for diagnosing pneumothorax because the symptoms associated with pneumothorax are also present in a number of unrelated clinical conditions such as cardiac ischemia, pneumonia, pulmonary embolism, esophageal spasm/reflux, and musculoskeletal strain. As a result, diagnosis of pneumothorax based on patient history and/or physical examination is very difficult and, in many cases, virtually impossible. For example, one study reported that physical examinations resulted in misdiagnosis in 42% of patients having a pneumothorax condition that arose from a penetrating chest wound.
Percussion is one common physical examination technique used by physicians to diagnose a variety of chest abnormalities. Most studies of percussion rely on qualitative descriptions such as “dull” and “resonant” to describe the chest sounds resulting from a percussive input to the patient's chest. Reported percussion response waveforms of a normal chest are typically 20 milliseconds (ms) long and contain an initial spike followed by a decaying waveform with spectral peaks in the 70 Hertz (Hz) to 200 Hz range. Using percussion, skilled physicians have noted “hyperresonance” as an acoustic phenomenon that is often heard in patients having a pneumothorax condition. In addition, acoustic asymmetries with large pneumothoraces have been reported when manually percussing both clavicles in turn while auscultating (i.e., listening to) the sternum. In any event, despite widespread belief in the usefulness of percussive techniques, uncertainty of its diagnostic capability exits because of the inherent dependence on the skill of the operator and their personal perception of the sound qualities of a patient's chest response.
Misdiagnosis of pneumothorax may also occur when using CXRs and CT due to large bullae and cysts within the lung or pleural space, patient clothing, tubing, skin folds, and chest wall artifacts. Additionally, with CXRs, patients are exposed to potentially harmful doses of radiation. Unfortunately, the radiation problem is compounded by the fact that CXRs are often performed unnecessarily (which needlessly exposes patients to radiation) because physicians are unwilling to miss the diagnosis due to the life threatening nature of pneumothorax, the tendency of pneumothorax to progress rapidly to tension pneumothorax and the ease with which pneumothorax can be treated if detected. As a result, CXRs are ordered as a precautionary measure for many patients that do not actually have pneumothorax. Further, because each patient with pneumothorax is typically subjected to multiple CXRs to generate subsequent films that document relative improvement, it is estimated that the total number of pneumothorax diagnostic tests conducted each year in the U.S. may be hundreds of thousands.
To overcome the diagnostic limitations of CXRs and CT, patients may be placed in the upright or lateral decubitus positions, and/or end-expiratory exposures may be used instead. Unfortunately, these positioning maneuvers are typically difficult to perform on critically ill patients. In addition to patient positioning difficulties, a common limitation of CXRs and CT is the difficulty and danger of transporting a critically ill patient to the imaging suite and the lack of equipment and staff availability in a timely manner, which is typically the case at night or in remote areas (such as, for example, battlefield conditions, the scene of an accident, a bedside, etc.). Further, CXRs, CT and other conventional imaging techniques typically involve a significant amount of delay between the examination of a patient and the availability of diagnostic results. Such a delay may be unacceptable in many situations, particularly where the patient's condition is critical or life-threatening. Still further, as is commonly known, diagnostic techniques based on ultrasound suffer from a high false positive rate due to inherent limitations.
Some researchers have used zero radiation techniques that rely on external low frequency forcing to non-invasively diagnose lung. diseases other than pneumothorax. For example, Wodicka et al. [Wodicka G R, Aguirre A, DeFrain P D, and Shannon D C, Phase Delay of Pulmonary Acoustic Transmission from Trachea to Chest Wall, IEEE Transactions on Biomedical Engineering 1992; 39:1053-1059] and Kraman et al. [Kraman S S, Bohandana A B, Transmission to the Chest of Sound Introduced at the Mouth, J Applied Physiology, 1989;66:278-281] studied acoustic transmission characteristics from the trachea to the chest wall by introducing low frequency sound waves at the mouth and measuring the sound waves received at the chest wall. The Wodicka et al. study found that geometrical changes within the lung cause sound transmission times to be frequency dependent because different wavelengths of sound couple to different parts of the lung lining. The Kraman et al. study found that changes in the lung volume or the resident gas composition did not consistently alter the peak-to-peak amplitude or the peak frequency of the measured signal. On the other hand, Donnerberg et al. [Donnerberg R L, Druzgalski C K, Hamlin R L, Davis G L, Campbell R M, Rice D A. British J, Diseases of the Chest 1980;74:23-31] studied the sound transfer function in normal and congested dog lungs using a technique similar to that described by Wodicka et al. and found a consistent increase in the transmitted sound as the lung wet-to-dry weight ratio increased.
Another abnormal respiratory condition that typically occurs in patients in ambulances and operating rooms is the misplacement of an endotracheal (ET) tube within a patient's trachea. As is generally known, ET tubes are placed in patients to establish an open airway, deliver anesthetic agents, and/or to perform mechanical ventilation. Typically, when an ET tube is misplaced, it travels too far into one of the two main bronchi (i.e., left and right) and blocks the other bronchus partially or completely, thereby limiting or eliminating ventilation into the lung associated with the obstructed bronchus. ET tube misplacement may also occur after the ET tube has been initially properly placed. For example, the ET tube may spontaneously move due to movements of the patient and/or movements of the ventilator tubing attached to the ET tube. Additionally, an ET tube may be misplaced into the esophagus of a patient or may be misplaced as a result of extubation.
As is well known, ET tube misplacement is a leading cause of hypoxemia and death during the course of general operative anesthesia, obstetric anesthesia, and in the management of critically ill patients in the intensive care unit, emergency room, and emergency settings outside the hospital environment. For example, mainstem intubation of a unilateral bronchus (i.e., placement of the ET tube into a left or right bronchus) may occur in approximately one-third of emergency endotracheal intubations because such emergency intubations depend heavily on operator skill and the clinical environment. Esophageal intubation and dislodgment of an endotracheal tube from its proper position or location are also relatively common occurrences. Unfortunately, ET tube misplacements are typically not recognized until after a chest radiograph is analyzed by a physician and, as a result, may lead to significant cerebral injury and/or death.
The most reliable known method of detecting proper endotracheal tube placement or location is direct visualization of the endotracheal tube passing through the vocal cords. However, under certain circumstances (e.g., blind nasal endotracheal intubation or endotracheal intubation through a laryngeal mask airway), such direct visualization may not be possible. Additionally, redundant soft tissue, blood, or the endotracheal tube itself may obscure a direct view of the vocal cords.
Due to the difficulty associated with detecting ET tube placement or location via direct visualization, ET tube placement or location is typically checked using x-rays, physiologic or acoustic techniques. However, the time, cost and radiation exposure associated with x-rays limits the usefulness of x-ray based detection of ET tube location, especially when multiple or on-line monitoring of the ET tube location is desired.
Physiologic techniques such as pulse oximetry and end-tidal CO2 detection are commonly employed to detect endotracheal tube location. Specifically, detection of end-tidal CO2 by capnography, capnometry, or colorimetric analysis may be used to detect ET tube location. However, with CO2-based techniques detection results may be corrupted if the patient has ingested carbonated beverages or antacids or if pulmonary blood flow is diminished or absent during cardiopulmonary resuscitation. While various CO2 detection techniques can be useful in detecting location of an ET tube, these techniques are typically not reliable for detection of mainstem bronchus ET tube malpositioning and provide limited accuracy.
Still further, some researchers such as, for example, Wodika et al. noted above, have used acoustic techniques to detect ET tube placement or location. The acoustic techniques described by Wodika et al. typically use an acoustic generator attached to one end of an ET tube to send sounds through the ET tube. Sounds reflected at a tip of the ET tube and sounds reflected by the airways within the patient are detected by acoustic sensors located near the acoustic generator. An analysis of the incident and reflected sounds may be used to facilitate ET tube positioning and monitoring.